1. Field of the Invention
This invention relates to the internal combustion engine art and, more particularly, to an improved arrangement for controlling the compression ratio of the engine.
2. Description of the Prior Art
The theory of internal combustion engines states that thermal efficiency, which is directly related to fuel economy, is directly proportional to compression ratio.
Compression ratio (CR) is defined as the ratio of the total internal volume between the top surface of the piston and the bottom surface of the cylinder head of a cylinder when the piston is at bottom dead center (BDC) to the clearance volume of the cylinder when the piston is at top dead center (TDC). The space between the piston and the cylinder head at TDC is also known as the combustion chamber. EQU CR=Total vol at BDC/Clearance vol at TDC
There is always a clearance space enclosed by the piston top surface and the inner surface of the cylinder head when the piston is at TDC.
It is important to differentiate between compression ratio and compression pressure (CP), although they are directly related. Typical compression ratios of modern spark ignition engines are anywhere from 8 to 9.5. The compression ratio for a particular spark ignition internal combustion engine design is selected after a determination of what safe compression pressures the engine can handle without the fuel mixture detonating prematurely.
The CP at part throttle will be lower than at full throttle. Thus, CR is limited by the maximum (full throttle) CP. This limitation, a) hinders the use of very lean mixtures for emission control, and, b) places an undesirable limitation on the theoretical efficiency of the engine.
But an automobile is driven mostly at part throttle. During partial throttle operation, the CR may be safely increased without exceeding maximum safe CP. An an increase in engine efficiency and a decrease in emissions may be realized if the CR is varied in a manner so that CP remains constant near a preselected value. To keep the CP level, the engine compression system must adapt to changing operation and external conditions such as load, speed, etc. and change the compression ratio therein.
A solution to providing variable CR is to somehow control the clearance volume at TDC.
Several variable compression ratio systems have attempted to provide a quiet, stable, controllable arrangement for changing the compression ratio in internal combustion engines. U.S. Pat. No. 4,516,537 teaches the use of a variable position sub-piston under hydraulic control. The '537 patent describes the inherent problem of a number of prior solution of the back-flow of the hydraulic fluid under the intense back pressure of the internal explosion of fuel and air. The prior art systems did not work as expected because the regulation of the compression ratio is accompanied by too large of an error imposed by the intense explosion pressure.
The '537 patent attempts to solve the problems by only moving the sub-piston during the intake and exhaust strokes of the engine. The opening and closing of a check valve is used to activate the movement of the sub-piston. However, '537 discloses that the sub-piston will be forced to a slightly rearward position during the intense power explosion. Such intermittent movement results in noise, vibration and control instability.
A disclosure in the Japanese patent No. 88926/81 attempts to solve some of the problems by introducing a hydraulic cylinder with a plunger mounted to be co-axial with the piston rod of the sub-piston used to vary the compression ratio. However, the system results in a stepwise control of the compression ratio which introduces a large error resulting in knock and erratic performance.
Other attempts such as U.S. Pat. No. 2,163,015, and similar U.S. Pat. Nos. 2,040,652 and 2,970,581, have attempted to replace the hydraulic control with a mechanical cam (which in this case was still under hydraulic control). However, the cam does not solve the problem of providing a "solid" configuration under ignition pressure. The geometric design of a cam introduces a lever and fulcrum into the physics of the system. The axis upon which the cam is mounted is never in line with the force vector. Thus there is always a moment force around the axis contributing to movement, noise, error and inoperability.
Thus, there has long been a need for an improved arrangement for controlling the compression ratio of the engine which provides positioning of a means to vary the clearance volume of the cylinder at TDC.
It is also desired that the control means be continuously variable and vibration resistant to provide noise free and error free efficient operation of the engine.